1. Field of the Invention
This invention relates generally to current mirror circuits and more particularly to transistor current mirror circuits used in analog integrated circuit devices.
2. Description of Related Art
Transistor current sources used in analog integrated circuits are generally well known. Typically, such circuits have been utilized as biasing elements and as load devices for amplifier stages. The use of current sources in biasing can result in superior insensitivity of current performance to power-supply variations and to temperature. Furthermore, current sources are frequently more economical than resistors in terms of the die area required to provide bias current of a certain value, particularly when the value of the bias current required is relatively small. When used as a load element in transistor amplifiers, the high incremental resistance of the current source results in high voltage gain at low power supply voltages.
One type of known current source comprises a current mirror and is commonly used in both junction and field effect semiconductor technology. In its simplest form, a current mirror circuit consists of a resistor and two transistors whose currents are constantly proportional to one another. In order to provide a more accurate current source which is less susceptible to variations in output current and changes in supply voltages, cascoded current mirrors were developed. However, as is well known, cascoded current mirrors require more headroom, i.e., more voltage compliance for achieving a desired operating range, than simple current mirrors. Accordingly, well known beta helper circuitry was then added to reduce base current errors; however this type of circuitry increases the headroom requirement even more.
Accordingly, it is an object of the present invention to provide an improvement in circuitry used in connection with analog integrated circuit architecture.
It is another object of the present invention to provide an improvement in analog integrated circuit current sources.
It is yet another object of the invention to provide an improvement in the type of transistor current sources known as current mirrors.
And it is yet still another object of the invention to provide an improvement in cascoded current mirror circuits.
These and other objects are achieved by the addition of an emitter follower to a current mirror and, more particularly to a cascoded current mirror that uses beta helpers to reduce the mirror error due to base currents for restoring a Vbe of voltage compliance to the cascoded current mirror, and thus allows the use of a cascoded current mirror in applications where it could not otherwise be used due to voltage compliance problems.
In its broadest aspect, the invention is directed to a current mirror circuit, comprising: at least two semiconductor devices, and preferably two pairs of cascoded semiconductor devices, such as transistors, having first and second current conducting electrodes and a control electrode, connected together in a current mirror circuit configuration, wherein the control electrodes thereof are commonly connected together, the first current conducting electrodes being directly connected to a first supply voltage terminal and the second current conducting electrodes being connected to a second supply voltage terminal through respective electrical impedance elements for providing a reference or input current through one of said semiconductor devices and an output current through the other of said semiconductor devices and wherein the reference current and the output current mirror each other and have a constant ratio; a third and a fourth semiconductor device, each having first and second current conducting electrodes and a control electrode, connected between said at least two semiconductor devices to improve the voltage compliance thereof, wherein the control electrode of the third semiconductor device is connected to the second current conducting electrode of said one semiconductor device, wherein one of said first and second current conducting electrodes of the third semiconductor device is directly connected to the first supply voltage terminal, and the other of said first and second current conducting electrodes of said third semiconductor device is connected to the second supply voltage terminal through a load impedance and directly to the control electrode of the fourth semiconductor device, and wherein one of said first and second current conducting electrodes of the fourth semiconductor device is directly connected to the second supply voltage terminal and the other of said first and second current conducting electrodes of the fourth semiconductor device is connected to the first supply voltage terminal through a load impedance and directly to the control electrodes of said at least two semiconductor devices.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are presented for purposes of illustration only, since various changes, alterations and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.